The present invention relates to light-emitting diodes (LEDs), particularly optical means for producing various far-field light intensity distributions for LEDs.
Conventional incandescent lamps of less than 100 lumens output can be matched by the latest white LEDs, albeit at a higher price. At this low end of the lumen range, the majority of incandescent applications are battery-powered. It is desirable to have an LED suitable for direct installation in the place of a burnt-out flashlight bulb.
LED's can offer superior luminous efficacy over the conventional incandescent lamps used in battery-operated flashlights. Moreover, LEDs are far more tolerant of shock, vibration, and crush-stress. Although they currently cost more to produce than the incandescents, their lifetimes are ten thousand times longer. For the sake of efficacy flashlight bulbs are run hot so they typically last only a few hours until filament failure. Also, the prices of LEDs continue to fall, along with those of the control-electronics to handle variations in battery voltage.
Indeed, LED flashlights are commercially available already, but their optics have to be adapted to the geometry of light-emitting diodes, which only emit into a hemisphere. Conventional LED lamps are unsuitable for direct installation into conventional flashlights, both electrically and optically. LED lamps are electrically unsuitable because they are current-driven devices, whereas batteries are voltage sources. Typical variations in the voltage of fresh batteries are enough to exceed an LED's tolerable operating-voltage range. This causes such high currents that the Ohmic heating within the die exceeds the ability of thermal conduction to remove it, causing a runaway temperature-rise that destroys the die. Therefore, a current-control device must accompany the lamp.
Conventional LED lamps are optically unsuitable for direct installation into the parabolic reflectors of flashlights. This is because their bullet-lens configuration forms a narrow beam that would completely miss a nearby parabola. Using instead a hemispherically emitting non-directional dome, centered on the luminous die, gives the maximum spread commercially available, a Lambertian pattern, with a sin2 θ dependence of encircled flux on angle θ from the lamp axis. Since θ for a typical parabolic flashlight reflector extends from 45° to 135°, an LED with a hemispheric pattern is mismatched because it's emission falls to zero at only θ=90°. This would result in a beam that was brightest on the outside and completely dark halfway in. Worse yet, even this inferior beam pattern from a hemispheric LED would require that it be held up at the parabola's focal point, several millimeters above the socket wherein a conventional incandescent bulb is installed.
Another type of battery-powered lamp utilizes cylindrical fluorescent lamps. Although LEDs do not yet offer better luminous efficacy, fluorescent lamps nonetheless are relatively fragile and require unsafely high voltages. A low-voltage, cylindrical LED-based lamp could advantageously provide the same luminous output as a fluorescent lamp.
Addressing the needs above, U.S. patent application Ser. No. 10/461,557, OPTICAL DEVICE FOR LED-BASED LIGHT-BULB SUBSTITUTE, filed Jun. 12, 2003, which is hereby incorporated by reference in its entirety, discloses such LED-based lamps with which current fluorescent and incandescent bulb flashlights can be retrofitted. It often desirable, however, for LED lamps such as those described in U.S. patent application Ser. No. 10/461,557 to have other far-field intensity distributions of interest. Also, U.S. patent application Ser. No. 10/461,557 touched on the function of color mixing, to make the different wavelengths of chips 23, 24, and 25 of FIG. 2 of U.S. patent application Ser. No. 10/461,557 have the same relative strengths throughout the light coming out of ejector section 12. This assures that viewers will see only the intended metameric hue and not any colors of the individual chips. Previously, rectangular mixing rods have been used to transform the round focal spot of an ellipsoidal lamp into a uniformly illuminated rectangle, typically in cinema projectors. Generally, polygonal mixing rods worked best with an even number of sides, particularly four and six. With color mixing for LEDs, however, such rods are inefficient because half of an LED's Lambertian emission will escape from the base of the rod.
There is thus a need in the art for effective and optically suitable LED lamps with various far-field intensity distributions and have proper shaping of their transfer sections enabling polygonal cross-sections to be used.